1. Field of the Invention
The present invention relates to a video camera. More specifically, the present invention relates to a video camera which performs various controls according to a luminance condition of a field.
2. Description of the Prior Art
FIG. 1 is a block diagram of a prior art.
In FIG. 1, 1 is a taking lens system. 2 is an infrared ray cut filter, which cuts infrared ray that cannot be seen by the human eye, causes a noise, and interferes with a photographing. 3 is an iris. 4 and 5 are color separation prisms; 4 leads only green light to a G-charge coupled device (hereinafter charge coupled device will be referred to as CCD)6, and 5 leads mazenta light having passed through the prism 4 to a R/B-CCD7. The light having passed through the prism 4 enters the G-CCD6; a luminance signal of the green light is detected by the entire picture elements of the G-CCD6 which outputs a signal G relating to the entered green light. The light having passed through the prism 5 enters the R/B-CCD7, which detects red light and blue light with a corresponding picture element, respectively, and outputs signals R and B relating to the entered red and blue light as separate signals. The signals of G, R, and B detected by CCDs 6 and 7 are converted into video signals after the noises are reduced by a well-known correlation double sampling (hereinafter referred to as CDS) circuit 8, respectively. After the noises are further reduced at a low pass filter (hereinafter referred to as LPF) circuit 9, the video signals of G, R, and B are amplified to an appropriate intensity at an auto gain control (hereinafter referred as AGC) circuit 10, and are clamped to a predetermined level in a clamp circuit 11. 12 is a white balance circuit, which adjusts white balance in response to a color temperature by adjusting the gain of R and B channel.
13 is a pedestal adjusting circuit, which maintains a black level of the video signals at a predetermined level (blanking level), to be more specific, adds a bias voltage to an input signal so that an output signal becomes zero when an input signal becomes zero.
A gamma correction circuit 14 corrects a gamma value in accordance with the characteristics of a TV receiver. A knee correction circuit 15 has input/output characteristics shown with a solid line in FIG. 13.
16 is a matrix circuit, which generates a luminance signal Y and color difference signals R-Y and B-Y from the R, G, and B signals. Thereafter, the color difference signals R-Y and B-Y are balanced-modulated in a modulation circuit 17, and then they are added to each other at an adder 50, and thereafter a color burst signal is added to them at an adder 51 to form a chrominance signal. Then, at an adder 53, they are added to a luminance signal Y to which a synchronizing signal has been added at an adder 52, and are outputted as composite video signals.
18 is a non additive mixing (hereinafter referred to as NAM) circuit as shown in FIG. 3, which inputs the signals of each of R, B, and G channels of the CDS circuit 8 and outputs only a signal having a maximum value of the R, B, and G signals.
41 is a detection circuit which smoothes a signal outputted by the NAM circuit 18.
An iris driving circuit 21 controls the iris 3 so that the level of a signal detected by the detection circuit 41 becomes constant.
When a rearlight correction switch 40 is closed, a predetermined value is added to the outputs of the detection circuit 41 at the adder 29. Therefore, in this case, the iris 3 is opened wider than when the control is performed based on only the output of the detection circuit 41, so that the taken image of the main subject against the light becomes brighter.
To improve the reproducibility of a luminance, a knee correction circuit 15 compresses a luminance difference of a wide dynamic range into that of a narrow dynamic range which can be reproduced on a TV screen. The compression is performed by compressing a gradation of a high luminance area. Since the obtained luminance difference of a video signal differs according to the luminance distribution condition of a field, it is preferable to change the compression method according to the luminance distribution condition. However, in the gradation compression method of the conventional knee correction circuit 15, for example, as shown by a solid line in FIG. 13, the range with an input signal intensity of 100% to 300% is set to be compressed into a range with an output signal intensity of 100% to 120%. In addition, the gradation compression starting point and the compression rate are fixed, respectively. However, since the luminance difference between a main subject and the background largely differs according to a field condition, with only one compression method, it is difficult to correct a reproduction gradation and therefore the reproduction gradation deteriorates. Especially, in a shooting of a scene including a main subject against the light or a main subject illuminated against the dark, for example, a person directed a spotlight, the above problem is arisen. There is another well-known gradation compression method in which a gradation compression starting point decreases as a peak value of a video signal increases. However, this method also cannot cope with a shooting of a scene including a main subject against the light or a main subject illuminated against the dark since the gradation compression is not changed according to a luminance distribution condition of the field.
Since the iris 3 is closed in by the iris driving circuit 21 when the average luminance of the field is high, a main subject becomes black in a shooting of a scene including the main subject against the light due to an insufficient exposure. To prevent this, the iris 3 is adjusted based on the main subject in a shooting of a scene including the main subject against the light. Even in this case, the light from the background intrudes into the main subject because of a light reflection on a lens surface, etc. and the subject becomes whitish, so that the image becomes less clear. For example, as shown in FIG. 11A, when a black subject is shot against the light, the center of the image plane where a main subject is situated becomes black and its surroundings where the background is situated becomes bright. However, since an internal reflection of an optical system causes a flare to increase a luminance of a subject image, the luminance level of the subject is increased by the flare, as shown in FIG. 11C, by the amount Vf more than the actual luminance level (black level) Vd shown in FIG. 11B. In correcting such a phenomenon, an excellent reproducibility of an image cannot be attained only by opening the iris 3 according to a luminance of a main subject.
The iris driving method comprising an average detection by the detection circuit 41 is directed to a control by which the average brightness of an entire image plane becomes an optimal luminance. In other words, the iris 3 is controlled so that a detection voltage of an image plane where the total area of high light parts is the same as that of black parts becomes a predetermined reference value. However, according to this method, an output of the image sensor (CCD) is apt to decrease, for example, in shooting an image plane of single color. As a result, the Signal to Noise (hereinafter referred to as S/N) ratio deteriorates. Another iris driving method is to use a peak detection. In this method, the output of the image sensor relating to an image of single color can be increased. However, since the iris 3 is controlled according to a maximum peak value regardless of the size of the area having a peak level, even when a high luminance part is in a small area, the iris is controlled according to a luminance level of the high luminance part in the small area. Consequently, the signal level in the wide area becomes low and therefore the entire image plane becomes dark, which deteriorates the S/N ratio.
A video camera usually shoots a frame of an image plane at every 1/30th of a second. To prevent flickers on a TV screen, an image plane for one frame is interlace-scanned: the image plane is divided into two image planes (fields), each field is scanned in 1/60 second, and a frame of a complete image plane is composed of two fields. When a CCD is employed as an image sensor, there are two methods, that is, a field accumulation mode and a frame accumulation mode, to obtain a video signal constituting the two fields. In the frame accumulation mode, the image of A and B fields are alternately shot. Here, A field is defined as a field consisting of odd scanning lines and B field is defined as a field consisting of even scanning lines. In a field accumulation mode, a field is constituted by combining two vertically adjoining image element signals into one image element signal. The images of the A field and the B field are alternatively shot by differentiating the combination of the adjoining picture elements by vertically shifting them as shown in FIG. 17 so that the images of the A and B fields obtain the same effect as the effect that of the image obtained by an interlace scanning. In the field accumulation mode, since the output signal of each image element is derived for 1/60 second, the light charge accumulation time is half that of the frame accumulation mode. Consequently, a dynamic range can be doubled.
However, in a place where there is only a little incident light, since the accumulation time is short, the S/N ratio is inferior to that of the frame accumulation mode, and the image quality deteriorates. In addition, since the output of each image element in response to the same quantity of the incident light is half that of the frame accumulation mode, the outputs of two vertically adjoining picture elements is combined to obtain a video signal having the same level as that of the frame accumulation mode. Consequently, a vertical resolving power decreases. On the contrary, in the frame accumulation mode, since a horizontal scanning line on an image plane is composed of an output of a picture element line in a horizontal line of a CCD, a vertical resolving power is superior to that of the field accumulation mode. However, since the light charge accumulation time is twice that of the field accumulation mode as described above, a subject luminance at which an accumulation electric charge starts to overflow is half that of the field accumulation mode, so that the dynamic range becomes narrow. Consequently, when a luminance difference is small, for example when a shooting is performed in a lighting from the front or in a room, by using the frame accumulation mode, an image signal with an excellent reproducibility is obtained. When a luminance difference is large, for example when a shooting is performed against the light, outdoors where there is a shade, or indoors where the background includes an outdoor view (high luminance part), by using the field accumulation mode, an image signal with an excellent reproducibility is obtained.
However, conventionally, the above-described mode selection has not been adopted; a shooting mode has been fixed in each type of a video camera, respectively.
In a video camera, to obtain a clear image plane, the brightness of an entire image plane is so controlled as to be maintained at a predetermined level by an iris and an AGC circuit, etc. That is, a signal with the greatest intensity of the video signals R, B, and G is selected at a NAM circuit 18, and then the selected signal is smoothed at a detection circuit 41. By the smoothed signal, the iris 3 is controlled through the iris driving circuit 21 so that the smoothed signal maintains a fixed intensity. However, such control method has a disadvantage that a main subject becomes dark under a shooting against the light because the background (high luminance part) largely affects the brightness of a projected image. Moreover, if the rearlight condition is corrected by only an amplification control of the AGC circuit, the CCD operates at a low level with respect to a subject against the light. Consequently, the S/N ratio with respect to a main subject deteriorates because an operation level of the CCD is not controlled by the iris 3, even if the CCD could be operated at a sufficiently high level with the iris opened to its opened end.